Upper respiratory tract infection (URTI), influenza like illness (ILI) and acute gastrointestinal infection (AGI) are common in the U.S. and are associated with significant morbidity and economic burden. Accumulating evidence indicates that vitamin D may play an important role in immune function;epidemiologic and clinical studies have documented an association between lower vitamin D levels and increased risk of several infectious diseases, particularly those of the respiratory tract. We propose to evaluate the effect of supplementation with vitamin D on the incidence and duration of common respiratory and gastrointestinal infections by adding appropriate symptom surveillance to our ongoing randomized, placebo-controlled double blind trial testing the efficacy of supplementation with vitamin D (1000Iu daily) or calcium (1200 mg daily) for the prevention of colorectal adenomas. The trial has completed recruitment, and will have over 1000 subjects still on study treatment and potentially evaluable for symptom surveillance over at least one winter season. Subjects have complied well with the study protocol, and the trial has extensive data already collected regarding lifestyle, 25-(OH) vitamin D levels, and major medical events. In the research proposed here, subjects from the parent trial will be recruited to complete monthly symptom diaries regarding URTI, ILI and AGI. The diaries, either in paper form or web- based, will be will be completed daily and returned monthly to the Study Coordinating Center. Subjects will be instructed to record respiratory and gastrointestinal symptoms for up to 12 months. Using pre-defined definitions of URTI, ILI and AGI, we will adopt as endpoints the number of episodes of each type of illness and the total number of days of affected by the illness. Primary analyses will focus on URTI;exploratory analyses will address ILI and AGI. We project that will be able to recruit at least 812 subjects into this effort, more than half of whom will be monitored for a full year. We estimate we will have greater than 80% power to detect a reduction of 20% or more in the incidence of URTI and even greater power to assess effects on number of days affected by these infections. PUBLIC HEALTH RELEVANCE: Colds, influenza and acute gastrointestinal infections are common occurrences in the United States, and are associated with a significant health and economic burden. Accumulating evidence indicates that vitamin D is important for immune functioning and that higher vitamin D levels may protect against these illnesses. In the study proposed here, we will evaluate the ability of supplemental vitamin D to reduce the risk of developing those infectious disorders.